Many modern elevators are equipped with sensors that provide load signals to the elevator controller indicative of the elevator car weight. The measured weight of the elevator car can be used by the controller to precisely pretorque the motor prior to releasing the brake, to implement special dispatching strategies such as bypassing hall calls when the car is full, or indicating an overload condition. For bypassing hall calls or indicating an overload condition, the load signal is compared with a predetermined load value stored in the elevator controller.
All sensor systems are subject to drift over long periods of time, resulting in the need for calibration. In the context of elevator car load weighing, sensor drift can result in jerky starts when the motor is incorrectly pretorqued. This can also result in the improper implementation of the special dispatching strategies such as hall call bypass when the car is not fully loaded or vice versa; for instance, when the load signal is larger than it ought to be, or in the failure to implement hall call bypass when the load signal is smaller than it ought to be.
The usual method of calibrating the load sensors is for service personnel to bring calibration weights to the building, and measure the sensor signals under different load conditions.